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KNMI for the Caribbean Netherlands
Volcanoes in the Dutch Caribbean
Glossary
Active Volcano: A volcano that is erupting or a volcano that is not presently erupting, but that has erupted within historical time and is considered likely to do so in the future. Mnt. Scenery and the Quill are active volcanoes.
Ash cloud: fragments, measuring less than 2 mm, of pulverized rock, and volcanic glass expelled from the volcano during an eruption. Ash clouds can reach 10-20 km altitude. Ash is very sharp and abrasive and can therefore be very harmful for people when they accidentally inhale it. Ash usually drifts depending on the wind direction, after which it is deposited on the ground. Ash clouds cause closures of airspace as ash is very damaging to airplane engines. Large quantities of ash can cause weak roofs to collapse.
Ashfall: Volcanic ash that has fallen through the air from an ash cloud. A deposit so formed is usually well sorted and layered.
Bomb: Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.
Earthquake swarm: When in a local area several earthquakes occur within a short timespan (hours to weeks). Usually the same process is responsible for the earthquakes and they stop when this process finishes.
Effusive eruption: A type of volcanic eruption in which lava steadily flows out of a volcano onto the ground. Mnt Scenery had at least one effusive eruption, which formed the lava flow which can still be seen today in the Northeast.
Explosive eruption: An eruption wherein magma is violently fragmented and rapidly expelled from a volcano. The pyroclastic flow deposits found on Saba and St. Eustatius are testimony to its past history of multiple explosive eruptions.
Fumarole: A vent or opening through which issue steam, hydrogen sulfide, or other gases.
Global Navigation Satellite System (GNSS): Instruments which use a constellation of satellites providing signals from space that transmit positioning and timing data to GNSS receivers. The receivers then use this data to determine location. GNSS is used on Saba and St. Eustatius to monitor ground movement. Ground movement may signify a change in volcanic activity when magma movement into the volcano causes swelling of the edifice.
Harmonic Tremor: Continuous long period vibrations typically associated with the underground movement of magma. Often seen just before or during an eruption.
Intrusion: The process of emplacement of magma in pre-existing rock.
Lahar: A torrential flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow. This type of volcanic deposit is often formed when freshly deposited ash is mixed with heavy rainfall.
Lava flow: An outpouring of lava onto the land surface from a vent. On Saba one can easily recognise an old lava flow running down the slopes from Hells Gate to the Airport.
Long period earthquake: A type of earthquake whereby the ground moves with a period longer than 1 second. On a volcano this type of earthquakes often signifies the presence of magma and may be a precursor to an eruption.
Magma Chamber: The subterranean cavity containing the gas-rich liquid magma which feeds a volcano. Not much is known about the magma chambers of Mnt Scenery and the Quill, but based on the rocks found on the islands, it is clear that magma chambers exist.
Magnitude: A number representing of the amount of energy released by an earthquake, determined by measuring earthquake waves on standardized recording instruments (seismometers) The number scale for magnitudes is logarithmic rather than arithmetic. Therefore, deflections on a seismograph for a magnitude 5 earthquake, for example, are 10 times greater than those for a magnitude 4 earthquake, 100 times greater than for a magnitude 3 earthquake, and so on.
Magmatic eruption: An eruption driven by the decompression of the gas within the magma.
Phreatic eruption: An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.
Phreatomagmatic eruption: An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma.
Pyroclastic flow: Lateral (downward) flowage of a turbulent mixture of hot gases and unsorted volcanic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles an hour.) The deposit which is formed by this process is called a pyroclastic flow deposit. This type of deposit can be found everywhere on Saba, most of the island is constructed by them.
Rockfall: An avalanche of loose rock. Often occurs after weathering or erosion destabilizes the rock face. Saba is prone to rock falls due to the steep topography of the island.
Satellite observation: Observations made from space. These can vary from gasses emitted by the volcano (by the TROPOMI instrument for example), to temperatures (by the MIROVA detection system for example) to cm-scale deformation (by SENTINEL for example).
Seismometer: An instrument that measures ground motion, for example caused by earthquakes. These earthquakes can be caused by tectonic movements or volcanic activity. On Saba and St. Eustatius currently most observed earthquakes are due to tectonic movements of the subducting plate. On each island three seismometers are operational.
Stratovolcano: A volcano composed of both lava flows and pyroclastic material. Mnt. Scenery and the Quill are both stratovolcanoes.
Tephra: Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.
Vent: The opening at the earth's surface through which volcanic materials arise.
Volcano tectonic earthquake: An earthquake induced by rock breaking due to the movement (injection or withdrawal) of magma. Volcano-tectonic earthquakes don't indicate that the volcano will be erupting soon but when their frequency increases it is a sign the volcano is active.
Volcanic system: The complete volcano, including parts which can be seen, such as the crater (vent) and the flanks of the volcano, and parts located below the surface, such as the conduit (pipe through which the magma erupts), magma chamber (space where the magma resides before an eruption) and connecting passageways.
Volcanic unrest: When the volcano exhibits signs which deviate from “normal” for that volcano. The signs could be for example the swelling of the surface, the appearance of earthquakes or the emission of gasses. Volcanic unrest can be short lived (hours) to long lived (months to years) and not all volcanic unrest leads to an eruption.
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